What is the chemistry of building materials?

Calcium oxide, CaO, usually called ‘lime’, reacts with water to form calcium hydroxide. It is the cheapest of all bases. Builders use enormous quantities of it for mortar and plaster.

What do we mean by lime? It really means calcium oxide. Other names for this quick compound are quick lime and unslaked lime.

Quicklime is a white solid that reacts violently with water. If you add just a little water to a lump of quicklime, you will be surprised at the result. The lime cracks, swells up, gives off steam, and acts as though it were alive. That is why it is called ‘quick’.

How is lime made? It is made by heating limestone in a special kind of furnace called a ‘kiln’. Lime must be stored in a dry place. Otherwise it will absorb moisture.

Mortar is a pasty mixture of slaked lime, sand and water. A little cement may be added to it, especially for underground foundations. The mortar quickly sets into a hard, rocklike solid.

There are three reasons why it hardens: 1) it loses water be evaporation, 2) carbon dioxide from the air unites slowly with the calcium hydroxide to form a mass of calcium carbonate, 3) the calcium hydroxide also unites very slowly with some of the small particles of sand, forming calcium silicate.

What is plaster? We usually put plaster on the walls or ceilings of our homes in two coats. The first, or undercoat, is much like water, except that it has hair or wood fiber added to it. This makes it hold together better until it has hardened. The second, or finish coat, is a mixture of calcium hydroxide, water and a powder called “plaster of Paris”. The finish coat dries quickly on a smooth, hard surface.

How do we make cement? We make cement from limestone and clay. These are cheap and plentiful raw materials. However, it takes a great deal of power and fuel to make cement, besides big, heavy machines for heating and grinding the raw materials. First limestone and clay are ground and mixed together. Then the powder mixture of raw materials is charged in the upper end of a cement kiln that revolves slowly. Fuel oil or powdered coal blown in at the lower end of the kiln is burned to supply the heat needed to begin to melt the mixture. At this temperature the mixture forms small lumps about the size of peas. The lumps drop out of the lower end and go to machines that grind them into a powder. This makes the finished cement.

The finer the grinding the harder the concrete we can make from cement.

What is concrete? We make concrete by mixing cement, sand, gravel and water together in the right amounts. The concrete is poured into forms that hold it in place until it hardens. The hardening is probably due to crystals forming in the concrete. They stick together and make a very hard artificial stone. Sometimes we embed steel rods in the cement mixture to make it stronger. This is reinforced concrete. In the building industry, cement was used with steel in foundations. It was mixed with sand and gravel, and reinforced with steel for floors and floor arches. In this form it became a new material – reinforced concrete. (3200)

Aluminium in structures

Aluminium is a metallic element extracted from bauxite. Silvery white, corrosion-resistant and extremely light in weight, ductile and malleable, it can be spun, pressed or cast. Discovered by Sir Humphrey Davy in 1807, it was not manufactured until a suitable flux was found in 1890. Probably the first example of large scale structural use of aluminium was in 1933 when the floor steelwork of a large road bridge in Pittsburg, Pennsylvania, was replaced with aluminium and the resulting saving in dead weight – about 1 ton/ft run – enabled the bridge to carry with greater safety the increased loads of modern traffic. Apart from the construction of aircraft, aluminium has thus a structural history of about only half a century. Extensive use of aluminium in buildings such as aircraft hangars did not occur until about 50 years ago.

In many ways aluminium has been slow in making progress, mainly because of its cost; it is produced by electrolytic means which requires hydro-electric power. About 10 units of electricity are required to make 1 lb (libra – pound – 0,454 kg). New reduction plants of large capacity have been coming into service in many parts of the world and these provide increased production with improved efficiency. The use of aluminium in structures may well expand in corresponding manner as simultaneous advances are being made with the development of improved aluminium materials and products.

The advantage of lightness - one-third the density of mild steel with nearly the same strength – is particularly of value where weight saving is of importance – in lift bridges, long span roofs, dome roofs, crane jibs, and in a wide range of moving or portable structures. (1600)

Plastics

Plastics are artificial substances containing natural or synthetic polymers, usually divided into two classes – thermoplastics and thermosets – which require different jointing methods in building. They can be moulded under pressure and usually set in an irreversible form on cooling.

Plastics were invented in the middle of the 19^th century but they were manufactured in quantity only in 1930s mainly for small household utensils. After the Second World War, when they were used in aircraft and their durability was proved, plastics were employed in buildings for wall and ceiling cladding, joinery and stucco mouldings, tubes and gutters, often in imitation of wood and metal. Sandwich panels with foam plastic cores which are lightweight and provide insulation were much employed for internal walls and high-strength moulded panels reinforced with glass-fibre for larger wall and roof components. Although they continued to be used mainly as substitutes for traditional materials, plastics were exploited to create new architectural forms.

The manufacture of plastics is, however, a source of pollution and as they cannot be recycled nor are they biodegradable, their use is discouraged by advocates of environmental architecture. (1200)

Stonehenge (c. 1900 B.C. – c. 1400 B.C.)

Stark on the windy plain of Salisbury stands a huddle of rough-hewed blocks, which is a prehistoric sanctuary near Amesbury, South England, 130 km west of London. This mysterious relic of ancient Britain is Stonehenge – place of the “hanging stones”.

Nowhere in Western Europe can be found a monument quite like it. Stonehenge has two settings of sarsens, a 97 foot-diameter circle that once held 30 columns, overlaid with a continuous lintel and an inner horseshoe of even grander blocks, some 200 feet tall and weighing more than 40 tons a piece.

Among these sarsen formations stand a secondary circle and horseshoe of bluestones.

No one can look up at these ponderous blocks without wondering how a primitive people with scant engineering knowledge could have brought them to this lonely spot and erected them. Antiquarians believe that as many as 1500 men could have laboured for ten years transporting all the immense stones across Wiltshire.

Even more perplexing questions occur. What about the fashioning of Stonehege collonades? How did men who never knew steel shape this tough, dense sandstone. For the tall sarsens are not only cut into regular oblong columns, but are capped by lintels, joined to the uprights by tenons and mortises, and the heavy lintels themselves are joined by a crude tongue – and - groove bond. (1400)

The Tower of London

The Tower of London was first built by William the Conqueror for the purpose of protecting and controlling the city. It covers an area of 18 acres within the Garden rails.

The present buildings are partly of the Norman period (period of medieval history in Britain starting from the Norman conquest in 1066, marked by the consolidation of the feudal system in Britain), but architecture of almost all the styles which have flourished in England may be found within the walls. In the past the Tower was a fortress, a palace and a prison and housed the Royal Mint, the Public Records and for a short time the Royal Observatory. For six hundred years, from the thirteenth century to the nineteenth, it also housed the Royal Menagerie, the predecessor of the London Zoo.

The oldest and the most important building is the Great Tower or Keep, called the White Tower. It is somewhat irregular in plan, for although it looks so square from the river its four sides are all of different lengths, and three of its corners are not right angles. The west side is 107 feet from north to south. The south side is 107 feet from north to south. The south side measures 118 feet. It has four turrets at the corners, three of them square, the fourth being circular. From floor to battlements it is 90 feet in height. The interior is of the plainest and sternest character. Every consideration is subservient to that of obtaining the greatest strength and security. The outer walls vary in thickness from 15 feet in the lower to 11 feet in the upper storey. (1500)

The Crystal Palace

The building made to contain the Great Exhibition in Hyde Park was later called the Crystal Palace, notable for having introduced a revolutionary principle into architecture, followed up but slowly in the 19^th century, but finally adopted in the 20^th century to the near exclusion of traditional methods. It was designed by Joseph Paxton who was neither architect, nor engineer who had made extensive experiments in the construction of large greenhouses. What was required was a hall of extraordinary dimensions, yet one that could be dismantled with a minimum of expense, leaving the amenities of the park unscathed.

The essential members were cast-iron columns with socketed heads that could be fitted one into the other and carried upwards for an indefinite number of stages. They were horizontally braced by cast-iron girders. The walls and roofs were of large panes of glass fixed by wooden sash-bars. The building followed the elementary plan on which the ancient basilica was based – a high nive with double side aisles and gallery, but with flat roofs at the three stages. The whole building was 1848 feet long. Foundations were of concrete.

All the world came to see this enormous metal-framed and also prefabricated building that was so much bigger than any cathedral and had a total exhibiting surface of 21 acres.

The Crystal Palace fulfilled all the requirements demanded of it. In 1853 it was dismantled without difficulty and rebuilt at Sydenham [ ] (large residential district in the south of London) where it housed another great International Exhibition in 1860. Thereafter, it continued in being as a stately pleasure dome until it was accidentally destroyed by fire in 1936. It was an epoch-making structure and it gave the world the first hint of the advantages to be gained in a metal-framed building. (1800)